[TGE] Amend block reward token handling

[ahtotruu]

Update the block reward token ID handling introduced in #1 according to the spec updates in unicitynetwork/unicity-yellowpaper-tex#4 and unicitynetwork/unicity-yellowpaper-tex#5.

As far as the PoW node is concerned, the differences are as follows:

• the node needs to take an additional configuration parameter: the ID of the network where the reward token is to be minted;
• the reward token ID is generated in two steps:
  • first an unpredictable and unique salt is generated (can be done just like the token ID is generated now);
  • then the token ID is generated by hashing the salt and the network ID;
• then the token ID is committed via the two-leaf hash tree;
• for reward token minting, the network ID and the salt must be saved.

See also unicitynetwork/state-transition-sdk-js#115.